Improve ergonomics of `tox` lint jobs

[jakelishman]

This fixes the lint job for tox>=4, which uses whitelist_externals is removed in favour of allowlist_externals. This also makes the lint and black runs skip installation of the package, which makes them much faster to use while making changes, and these are static analysis tools, so don't need to install.

[coveralls]

Pull Request Test Coverage Report for Build 7844408343

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.568%

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Totals
Change from base Build 7833941903:	0.0%
Covered Lines:	16740
Relevant Lines:	17335

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💛 - Coveralls